1. Field of the Invention
The present invention relates to a transmission method, a transmitter and a receiver, which is preferably applied to a cellular radio-communication system such as a portable telephone system.
2. Description of the Related Art
A conventional cellular radio-communication system is constituted so that an area for which communication services are provided is divided into cells of a desired size, a base station serving as a fixed radio station is set in each of the cells, and a portable telephone serving as a mobile radio station performs radio communication with a base station in a cell in which the portable telephone is present.
In the above case, various communication systems are considered between a portable telephone and a base station. Typical ones of the systems are the code division multiple access (CDMA) system and the time division multiple access (TDMA) system.
The CDMA system is a communication system for transmitting a narrow-band modulated wave (primary modulation) by spectrum-spreading the frequency bandwidth of the wave (secondary-modulating) and thereby, widening the width up to several tens of times or more, in which every communication is performed by using the same radio carrier so that multiple access can be realized by assigning an independent spread code to each communication channel and thereby assigning the same wide frequency band to a multiplicity of communication channels. The receiving side can restore transmission information by reverse-spreading the signal of a desired channel, thereby recognizing signals of other channels as noises and extracting only a purposed primary-modulated wave, and demodulating the wave.
The transmitter and receiver of a cellular radio-communication system for transmitting or receiving a digital signal in accordance with the CDMA system are described below by referring to FIGS. 1 and 2. In this connection, the transmitter and receiver shown in FIGS. 1 and 2 are mounted on, for example, the base station of a portable telephone system or a portable telephone and used for the up communication from the portable telephone to the base station or the down communication from the base station to the portable telephone.
As shown in FIG. 1, symbol 1 denotes the transmitter of a cellular radio-system according to the DS(Direct Spread)-CDMA system (hereafter simply referred to as CDMA system) as a whole. In FIG. 1, a case is assumed in which a communication environment using the same frequency band for all adjacent cells, that is, the so-called state in which the number of repetitions of frequency is “1” is set and an information bit stream S1 is transmitted at a bit rate of 8K[bit/sec] desired by a user by using a bandwidth of 1.024 [MHz].
The transmitter 1 first inputs the information bit stream S1 of a bit rate of 8K[bit/sec] to an encoding section 2. The encoding section 2 generates a transmission symbol stream S2 of 16K[Coded bit/sec] encoded at an encoding rate of ½ by applying the convolutional encoding which is one of error corrections to the information bit stream S1 and rearranging the sequence of transmission symbols thereby obtained at random (rearranging of the sequence is hereafter referred to as interleaving) and transmits the stream S2 to a spread-code multiplier 3.
The spread-code multiplier 3 generates a transmission symbol stream S3 spread to 1024K[Chip bit/sec] by multiplying the 16K[Coded bit/sec] transmission symbol stream S2 by a spread code C1 having a spreading ratio (hereafter referred to as SP) of 64 supplied by a spread-code generating section 4 and supplies the stream S3 to a long-code multiplier 5. In this case, the spread-code multiplier 3 assigns the same frequency band to 64 channels by using 64 types of PN codes orthogonal to each other.
In this case, in the transmitter 1, the spread code C1 having an SP corresponding to the bit rate of the information bit stream S1 is assigned by the spread-code generating section 4 when channel assignment is requested. Therefore, when the bit rate of the information bit stream S1 is 16K[bit/sec], the spread code C1 having an SP of 32 is assigned.
The long-code multiplier 5 performs scrambling by multiplying the transmission symbol stream S3 by a long code C2 intrinsically set to each base station supplied from a long-code generating section 6 so that crosstalk does not occur even if the same spread code C1 is used for adjacent cells and transmits a transmission symbol stream S4 of 1024K[chip/sec] thereby obtained to a symbol mapping section 7.
In this case, because the number of repetitions of frequency is “1,” the same frequency band is used for adjacent cells and moreover, the same frequency band is used for the spread code C1. Therefore, by using the long code C2 intrinsically provided for each base station, the interference between adjacent cells is prevented. That is, in the case of a CDMA cellular radio-communication system, the spread codes C1 used for portable telephones are orthogonal to each other in the same cell. However, there is not orthogonal relation between adjacent cells.
The symbol mapping section 7 generates a transmission signal S5 showing each piece of symbol information by a phase value by applying the binary phase-shift keying (BPSK) modulation processing to the transmission symbol stream S4 successively input and transmits the signal S5 to a transmission circuit 8.
The transmission circuit 8 generates a transmission signal S6 of a predetermined frequency channel by filtering the transmission signal S5, then transforming the signal S5 into an analog signal, and multiplying the transmission signal transformed into an analog signal by a high frequency, and thereby frequency-converting the analog transmission signal into a desired frequency band (e.g. approx. 800 [MHz]), and amplifies the signal S6 to a predetermined power and thereafter transmits the signal S6 through an antenna 9.
Moreover, as shown in FIG. 2, the receiver 10 receives the transmission signal S6 transmitted from the transmitter 1 through an antenna 11 and inputs the signal S6 to a receiving circuit 12 as a reception signal S11. The receiving circuit 12 fetches a base band signal by amplifying the reception signal S11 up to a predetermined level and then, frequency-converting the signal S11, moreover fetches a BPSK-modulated reception signal S12 by filtering the base band signal and then, transforming the signal S12 into a digital signal, and transmits the signal S12 to a bit-stream extracting section 13.
The bit-stream extracting section 13 fetches symbol information by applying the BPSK demodulation processing to the reception signal S12 and transmits the symbol information to a long-code multiplier 14 as a reception symbol stream S13 of 1024K[chip/sec].
The long-code multiplier 14 receives a long code C3 same as that generated in the long-code generating section 6 at the transmission side from a long-code generating section 15 and performs descrambling by multiplying the reception symbol stream S13 by the long code C3. Thereby, the long-code multiplier 14 generates a reception symbol stream S14 of 1024K[chip/sec] same as the transmission symbol stream S3 generated at the transmission side and transmits the stream S14 to a spread-code multiplier 16.
The spread-code multiplier 16 generates a spread code C4 same as the spread code C1 generated by the spread-code generating section 4 at the transmission side with the spread-code generating section 17, reverse-spreading the spread code C4 by multiplying the reception symbol stream S14 of 1024K[chip/sec] by the spread code C4, and transmits a reception symbol stream S15 of 16K[Coded bit/sec] thereby obtained to a decoding section 18.
The decoding section 18 returns the sequence of reception symbol streams S15 to the original sequence by reversing the rearrangement performed in the encoding section 2 of the transmitter 1 (returning to the original sequence is hereafter referred to as deinterleaving) and the soft-decision Viterbi decoding is performed by considering the trellis of a convolutional code in accordance with a reception symbol stream thereby obtained and estimating the maximum likelihood state (so-called maximum-likelihood series estimation) out of all state transitions which can be used as data and thus and an information bit stream S16 of 8K[bit/sec] showing the data thus transmitted is restored and output.
As shown in FIG. 3, the TDMA system is, for example, a communication system of temporally classifying a predetermined frequency channel in accordance with frames F0, F1, . . . respectively having a predetermined time width, dividing the frames F0, F1, . . . into time slots TS0 to TS7 (in this case, 8 time slots) respectively having a predetermined time width, and using the frequency channel at the timing of the time slot TS0 assigned to a local station, and thereby transmitting a transmission signal, in which pluralities of communications (so-called multiple access) are realized with the same frequency channel to efficiently use frequencies. In the subsequent description, the time slot TS0 assigned for transmission is referred to as a transmission slot TX and a data block sent by one transmission slot TX is referred to as a slot.
In this case, the time slot TS0 is assigned to a user A, the time slot TS1 is assigned to a user B, the time slots TS2 and TS3 are assigned to a user C, and the time slots TS4 to TS7 are assigned to a user D. Thereby, transmission rates can be changed by changing the number of time slots to be used every user. Even in this case, however, a transmission rate of 8K[bit/sec] desired by a user is assigned to each physical channel (in this case, the total of 8 channels because there are 8 time slots) since the establishment of communication channels but the transmission rate for each channel is not changed under communication.
In this connection, in the case of the TDMA system, each of the time slots TS0 to TS7 is assigned to a predetermined frequency channel whenever it is actually transmitted by the transmission slot TX so that an assigned frequency channel is released whenever transmission is completed, and a frequency is effectively used by using a frequency channel only when thereby performing transmission.
Then, the transmitter and receiver of a cellular radio-communication system for transmitting or receiving a digital signal in accordance with the TDMA system are described below by referring to FIGS. 4 and 5. In this connection, the transmitter and receiver shown in FIGS. 4 and 5 are mounted on, for example, the base station of a portable telephone system or a portable telephone and used for the up communication from the portable telephone to the base station or the down communication from the base station to the portable telephone.
As shown in FIG. 4, symbol 20 shows the transmitter of a TDMA cellular radio-communication system for performing frequency hopping (FH) as a whole. Also in FIG. 4, a case is assumed in which a communication environment using the same frequency band for all adjacent cells, a so-called state in which the number of repetitions of frequency is “1” is set and the information bit stream S1 is transmitted at a bit rate of 8K[bit/sec] desired by a user by using a predetermined bandwidth.
The transmitter 20 first inputs an information bit stream S20 of 8K[bit/sec] to an encoding section 21. The encoding section 21 generates a transmission symbol stream S21 of 16K[Coded bit/sec] encoded at an encoding rate of ½ by applying the convolutional encoding to the information bit stream S20 and applying interleaving to a transmission symbol thereby obtained and transmits the stream S21 to a symbol mapping section 22.
The symbol mapping section 22 generates a transmission signal S22 showing each piece of symbol information by a phase value by classifying the transmission symbol stream S21 every predetermined number of bits in order to assign the stream S21 to the transmission slot TX and applying the BPSK (Binary Phase-Shift Keying) modulation processing to a transmission symbol stream thereby obtained and transmits the signal S22 to a transmitting circuit 23.
The transmitting circuit 23 generates a transmission signal S23 of a predetermined frequency channel by filtering the transmission signal S22 and thereafter transforming the signal S22 into an analog signal and multiplying the transmission signal transformed into an analog signal by a high-frequency signal, and thereby frequency-converting the analog transmission signal into a desired frequency band (e.g. approx. 800 [MHz]), and amplifies the signal S23 up to a predetermined power and thereafter, transmits the signal S23 classified in slots through an antenna 24 synchronously with the timing of the transmission slot TX.
Moreover, the transmitting circuit 23 is constituted so as to change frequency channels used every slot at random in accordance with a predetermined pattern (so-called frequency hopping) and thereby, reduce the influence of interference waves received from other types of communication.
Thus, in the case of the TDMA system for performing frequency hopping, though physical frequency channels are changed at random, one logical channel is assigned to a user and only physically-usable portions (frequency channel) of the logical channel are changed. Therefore, the logical channel assigned to each user at the establishment of communication is constantly used while communication is performed between a base station and a portable telephone.
Moreover, as shown in FIG. 5, a receiver 30 receives the transmission signal S23 transmitted from the transmitter 20 through an antenna 31 and inputs the signal S23 to a receiving circuit 32 as a reception signal S31. The receiving circuit 32 fetches a base-band signal by amplifying the reception signal S31 up to a predetermined level and thereafter, frequency-converting the signal S31 and moreover, fetches the BPSK-modulated reception signal S32 by filtering the base-band signal and thereafter, converting the signal into a digital signal, and transmits the signal S32 to a bit-stream extracting section 33.
In this case, the receiving circuit 32 changes frequency channels received in accordance with a hopping pattern same as that of the frequency hopping performed at the transmission side and thereby, accurately executes the receiving operation in accordance with the change of transmission-side frequency channels.
The bit-stream extracting section 33 fetches symbol information by applying the BPSK demodulation processing to the reception signal S32 and transmits the symbol information to a decoding section 34 as a reception symbol stream S33 of 16K[coded bit/sec].
The decoding section 34 deinterleaves the reception symbol stream S33 by reversing the rearrangement performed by the encoding section 21 of the transmitter 20, performs the maximum-likelihood series estimation in accordance with the reception symbol stream obtained as the result of deinterleaving and thereby, performs the soft-decision Viterbi decoding, and restores and outputs the information bit stream S34 of 8K[bit/sec] showing the data transmitted as the result of the soft-decision Viterbi decoding.
In the case of the CDMA cellular radio-communication system having the above structure, if a portable telephone currently transmitting undesired waves is present at a position very close to a base station in the up channel from the portable telephone to the base station, the undesired waves output from the portable telephone serve as interference components. To reduce the influence of the interference components, in the case of the CDMA cellular radio-communication system, both the base station and the portable telephone monitor the reception power (or the quality of the reception power) so as to control the transmission power by communicating the monitoring result each other.
Therefore, the CDMA cellular radio-communication system executes the so-called transmission power control purposing that the influence of interference components can be reduced without increasing the transmission power of undesired waves serving as interference components for other stations by performing communication with the minimum necessary transmission power.
Actually, the CDMA cellular radio-communication system detects the ratio between the desired-wave power when receiving a transmission signal from a portable telephone at a base station and the sum of the energy and thermal noises of interference components received by the base station (the sum is hereafter referred to as interference-wave power), that is, the signal-to-interference-wave-power ratio C/I and controls the detection result so that it becomes a value capable of withstanding a desired communication quality.
Moreover, in the case of the CDMA cellular radio-communication system, interference components produced due to calls generated in adjacent cells are averaged and provided for the calls in all cells in a local station so as to slowly influence them but only a specific call generated in a cell of the local station is not entirely influenced by the interference components. Thereby, the interference components produced due to calls generated in adjacent cells are determined as a certain averaged value. Therefore, when the transmission power in the cells of the local station increases up to a degree capable of ignoring the influence of interference components, it is possible to perform communication with no problem even if using the same frequency band in the base station of adjacent cells.
Therefore, the CDMA cellular radio-communication system is a communication system assuming that the energy of interference components received by a base station can be recognized to be almost constant (an averaged value) without instantaneously greatly fluctuating. Therefore, if the energy of interference components received by the base station greatly fluctuates, a portable telephone communicating with the base station must greatly fluctuate the transmission power.
Therefore, when the energy of interference component received by a base station suddenly increases, a portable telephone must increase the transmission power by a value corresponding to the increase of the energy. However, because a portable telephone is constituted so as to increase the transmission power in accordance with a power-up command sent from a base station if the energy of interference components extremely suddenly increases. Therefore, a time lag occurs before the transmission power is actually increased after receiving the power-up command and as a result, the communication between the base station and the portable telephone may be instantaneously broken.
In the case of an actual CDMA cellular radio-communication system, the transmission rate per user is approx. 14K[bit/sec] at most and the transmission rate is transmitted at a bandwidth of 1.23 [MHz]. In this case, the rate occupied by a channel assigned to one user for a bandwidth of 1.23 [MHz] (the rate is hereafter referred to as process gain) is approx. 87 (=1.23 [MHz]/14K[bit/sec]) and the fluctuation of calls for one channel (whether the fluctuation occurs) does not have a great influence on the entire system.
However, to transmit an information bit stream having a transmission rate of 400K[bit/sec] at a bandwidth of, for example, 4 [MHz], the process gain reaches 10 (=4 [MHz]/400K[bit/sec]) and thus, the influence of fluctuation of calls for one channel on the entire system cannot be ignored. Similarly, to transmit an information bit stream having a transmission rate of 800K[bit/sec] at a bandwidth of, for example, 4 [MHz], it is no longer possible to operate the system without considering the influence of fluctuation of calls for one channel on the entire system.
In the above case, if communication is suddenly started at a transmission rate desired by a user from the beginning of establishment of a communication channel (that is, if the rate of channels used among communication channels of the entire system is high), the transmission power must be increased correspondingly to the number of channels used. Thereby, a problem occurs that the interference value suddenly increases to affect other types of communication. Moreover, in this case, a time lag occurs in a mobile station before the transmission power is increased correspondingly to a power-up command sent from a base station. Thereby, a problem occurs that the communication between the base station and a portable telephone is instantaneously broken.
The same is true for a TDMA cellular radio-communication system for performing frequency hopping independently of a CDMA cellular radio-communication system. Therefore, when suddenly starting communication at a transmission rate desired by a user from the beginning of the establishment of a communication channel, problems occur that the interference value suddenly increases and the communication between a base station and a portable telephone is instantaneously broken because power control for controlling the increasing interference value cannot follow.